Classically electricity storage for PV panels is mostly designed for stand-alone applications In contrast, we focus in this article on houses connected to the grid with a small-scale storage to store a part of the solar power for postponed consumption within the day or the next days. In this way the house owner becomes less dependent on the grid and does only pay for the net shortage of his energy production Local storage solutions pave the way for many new applications like omitting over-voltage of the line and bridging periods of power-line black-out Since 2009 using self-consumption of PV energy is publicly encouraged in Germany, which can be realised by electric storage. This paper develops methods to determine the optimal storage size for grid-connected dwellings with PV panels From measurements in houses we were able to establish calculation rules for sizing the storage Two situations for electricity storage are covered: - the storage system is an optimum to cover most of the electricity needs; it is an optimum for covering the peak power need of a dwelling. After these calculation rules a second step is needed to determine the size of the real battery The article treats the aspects that should be taken into consideration before buying a specific battery like lead acid and lithium-ion batteries (C) 2010 Elsevier Ltd All rights reserved